Optical data storage systems utilize a disk onto which is affixed a recording medium for storing information in digital form. An optical head powered by a laser writes data on a recording medium by ablating and/or otherwise deforming the recording mediums reflectivity. Reading of the data then takes place as a function of sensing the light reflected from the nonmodified surface and the laser deformed surface. The deformations correspond to bit values, generally in a code format which facilitates the packing of the bits into the recording track of the optical disk. During a read operation the laser beam's power is lowered to a read intensity level and the beam is directed onto the optical disk somewhat identically to the beam that was used in the write mode. A detector within the system detects reflected light passing back through the systems optics and the detected signal is converted to an electrical signal the magnitude of which is a function of the reflection from the individual bit positions on the surface of the optical disk. To insure data integrity, it is necessary to verify the accuracy of the write operation to determine whether or not data was properly written to the individual bit positions. Two general verification techniques are known within the art. One is that individual bit positions are read after they have been written. This usually occurs in a second revolution of the disk. The first revolution being dedicated to the writing of the bits and the second revolution being dedicated to the reading and the comparing of the read bits with the written bits. Multiple beam optical heads can be used to provide the write beam and the read beam with the two beams being spaced apart a distance from each other such that the read beam can be reading the data prior to the second complete revolution of the optical disk. Such a system is perfectly satisfactory other than the fact that two systems almost identical in nature are required and of course the associated expense is approximately double what is needed for a single beam system.
A second type of verification technique known as the direct read during write has received some usage. There characteristics are that after the write signal is turned on the write detect signals, monitored immediately thereafter, are analyzed to determine the reflected properties of the surface of the disk.
A patent of particular interest for setting forth the state of the art is U.S. Pat. No. 4,680,594 entitled "Single Beam Direct Read During Write and Write Protect System for Use in an Optical Data Storage System" by R. Bracht. The system described in that patent is a direct read during write. The system detects the reflected light from the reflective surface as the write laser burns away portions of the recording layer to expose a nonreflective surface. The light that is reflected is detected and its characteristics observed for a time delay period which period permits the laser, if in the burn mode, to physically accomplish the destruction of the reflected surface. Such being the case the reflected light will commence at a relatively high value and will decrease to the value experienced when the nonreflective surface is exposed. By comparing the received signal from the detector with the desired write or bit positions it is possible to perform an accurate check of the writing on the surface of the disk.
Another patent of interest for its teaching is U.S. Pat. No. 4,428,075 entitled "Methods of Preformatting an Optical Disk" by R. L. Hazel et al. In the apparatus of that patent a multiplicity of laser beams are used, including a read-before- write beam along with a read-after-write beam and three beams that are used to selectively write across the width of a track.
Another patent of interest is U.S. Pat. No. 4,622,564 entitled "Optical Information Recording Apparatus" by T. Kaku et al. The apparatus disclosed in the patent utilizes two semiconductor lasers to provide two spot beams on the surface of the disk which are separated by a fixed distance. The primary function of such a configuration is to provide an error check immediately after the writing operation. The aforementioned Patent is an example of the dual laser, one high power and one low power, systems which perform the reading function almost immediately after the write function.